The present invention relates to a position sensing apparatus, and more particularly, to a position sensing piston/cylinder assembly.
Cylinder position sensing devices are known using various technologies. For example, an integrated sensor, such as a magnetostrictive type sensor (made by sold by MTS Systems Corporation and others), has been attached to an end of a cylinder and inserted into a bore in the rod. This sensor has fixed and moving parts within the rod. This requires a rather large hole to be bored in the rod and some means of fixing the sensing electronics on the end of the cylinder, as well as and means to transmit signals out from the barrel. Such sensors work primarily only with single ended rods. U.S. Pat. No. 5,115,195 describes a design using a short MTS magnetostrictive sensor with multiple magnets for measuring position of a moving body relative to a fixed body. However, it would be difficult and costly to incorporate such a sensor and magnets in a hydraulic cylinder.
U.S. Pat. No. 5,539,993 describes a system wherein a bar-code is placed on the outer surface of the rod and the position is determined with an appropriate bar code reader. It is also known to machine grooves into the outer surface of a cylinder rod, chrome plate over the grooves and then xe2x80x9ccountxe2x80x9d the number of grooves to determine position. This method can determine travel, but not absolute position in the event of cylinder drift in a non-powered state.
Certain cylinder position sensing devices include a linear Hall Effect sensor. But, typically, the Hall Effect sensor is not integrated into the cylinder. U.S. Pat. No. 5,201,838 describes a cylinder position sensing device using a pair of Hall Effect sensors and a single permanent magnet is mounted on a face of the piston head. However, such a design requires a very strong magnet and is likely to work only for a small cylinder with a limited stroke. UK patent GB 2108678 describes a design with a Hall Effect sensor and a magnet mounted outside of a piston cylinder housing which receives a stepped diameter member. However, this design requires a stepped diameter member and can only detect discrete positions corresponding to the different diameter portions of the member.
U.S. Pat. No. 6,253,460 describes a position sensing cylinder design which includes a tapered piston rod which is coated with a non-ferrous material. The sensor detects the distance from the sensor to the tapered rod through the non-ferrous coating. Such a design is difficult to manufacture because it is difficult and time consuming to apply such a non-ferrous coating to a tapered rod, and the outer surface of the coating must be uniform to assure a good seal. Also, the non-ferrous material must be hard enough to meet strength and load requirements, and it may be difficult to obtain a coating which is strong, yet pliable enough in order to build up the rod. Finally, with this design, debris or contaminants between the coating and the sensor may cause inaccurate readings.
U.S. Pat. No. 5,359,288 discloses a position detecting apparatus with a magnetic medium mounted on a movable member and a magnetic field sensor, such as a Hall cell, mounted on a fixed member. The magnetic medium has different areas with different polarities. However, it appears that such a design would yield inaccurate measurements if the movable member moves transverse to its axial dimension, and therefore such a design would not function accurately in a hydraulic cylinder where the rod may rotate or move perpendicular to its normal direction of travel.
Accordingly, an object of this invention is to provide a compact, robust and accurate cylinder position sensing assembly.
Another object of the invention is to provide such a cylinder position sensing assembly which accurately determines cylinder piston/rod position despite movement of the piston rod in multiple directions.
Another object of the invention is to provide such a cylinder position sensing assembly which accurately determines cylinder piston/rod position despite rotation of the piston rod relative to the cylinder housing.
A further object of the invention is to provide such a cylinder position sensing assembly wherein a hole need not be bored in the rod and wherein the sensor is located outside of the pressurized chamber of the cylinder.
These and other objects are achieved by the present invention, wherein a cylinder position sensing system includes a cylinder housing which slidably receives a piston with a rod or shaft element. The cylinder housing has an annular end piece which slidably receives the rod. First and second magnet members are mounted on or in the rod. The second magnet member is spaced apart from the first magnet member, and the spacing therebetween varies as a function of the axial position along the rod.
Preferably, a first axial groove is formed in the shaft element, and the first magnet member is mounted in the first groove. A second helical groove is formed in the shaft element and is spaced apart from the first groove. The second magnet member is mounted in the second groove. A magnetic field sensor is mounted in the cylinder housing so that the sensor generates an output signal as a function of a relative distance between the first and second magnet members as the second element moves with respect to the first element. Preferably, the rod includes a central cylindrical shaft and a hollow cylindrical sleeve mounted over and receiving the shaft. Preferably, the first and second grooves are formed in an inner surface of the sleeve, and the sleeve and the end piece are formed of non-ferrous material.